Ultrasonic imaging advantageously permits the real-time visualization of biological tissue structures within a human or animal body and, in addition, allows the non-destructive testing of materials. For convenience, a body or material to be imaged or tested is referred to herein as the object. In ultrasonic imaging an electronically-reconstructed or viewable image is typically generated by scanning or sweeping a pulsed ultrasound beam across the object to perform pulse echo reflection imaging of structures present within the object.
One problem with ultrasonic imaging, however, is that the reception of ultrasound echoes is subject to inherent acoustic noise. Reverberation echoes--i.e. second order reflections--constitute a strong and significant portion of this acoustic noise. The term reverberation refers to an ultrasound pulse that is reflected back and forth between internal tissue/material structures (i.e. internal reverberations) or between such tissue/material structures and the transducer surface (i.e. transducer-tissue/material reverberations). In image reconstruction or generation it is assumed that each echo comes directly from a scatterer that is hit by the propagating pulse emitted by the transducer. This assumption, however, results in the unfortunate misinterpretation of multiple-reflection echoes and thus constitutes acoustic noise.
A special transducer design incorporating a .lambda./2 matching layer is described in the Proceedings of the 19.sup.th International Symposium on Acoustical Imaging, at pp. 219-33 (1991). That transducer design is intended to minimize the reflection factor at the transducer surface so as to reduce reflections of received pulses at the transducer surface and hence reduce reverberation echoes.
A method of reducing reverberation echoes by dual frequency image subtraction is described in 12 IEEE Transactions On Medical Imaging, pp. 792-802 (1993). A particularly serious drawback to the dual frequency image subtraction method of reducing reverberation noise is that, in addition to internal reverberations, the received first order back-scattered echo signals are also attenuated.